The present invention relates to a semiconductor laser control circuit for adjusting a modulated laser power of each level to a target value when the output modulation of emitted laser radiation is performed among multi-value levels.
Examples of known recording mediums storing optically rewritable information thereon include phase-change storage media and magneto-optical recording media. In writing information onto a phase-change storage medium, an information layer of the medium is irradiated with a focused laser beam, thereby partially heating and fusing the information layer. The highest temperature the information layer can reach due to the heat applied thereto or the cooling process of the layer differs depending on the intensity of the laser radiation incident thereto. Thus, the optical characteristics of the information layer, such as the refractive index thereof, are locally modifiable by modulating the intensity of the laser radiation emitted. More specifically, if the intensity of the laser radiation is higher than a predetermined reference level, part of the information layer of the recording medium that has been irradiated with the radiation is rapidly cooled from an elevated temperature so as to be amorphized. If the intensity of the laser radiation is relatively low on the other hand, the irradiated part of the information layer of the recording medium is gradually cooled from an intermediate to high temperature and therefore crystallized. The amorphized part of the information layer of the recording medium is called a xe2x80x9cmarkxe2x80x9d, while the crystallized part is called a xe2x80x9cspacexe2x80x9d. That is to say, the mark and space have mutually different optical characteristics in terms of their refractive indices, for example. Accordingly, binary data is storable in the information layer of the recording medium by arranging the marks and spaces in a specific pattern. As used herein, the laser radiation for use in information recording will be called xe2x80x9cwrite radiationxe2x80x9d.
In reading out information stored on a phase-change storage medium, the information layer thereof is irradiated with a laser radiation beam with an intensity low enough to not cause any phase change in the information layer and a radiation beam, which is reflected from the information layer, is detected. As used herein, the laser radiation for use in information readout will be called xe2x80x9creadout radiationxe2x80x9d. The mark, or the amorphized part of the information layer of the recording medium, has a relatively low reflectance, while the space, or the crystallized part of the information layer of the recording medium, has a relatively high reflectance. Accordingly, by recognizing the difference in the amount of the radiation reflected from the mark and space, a read signal can be obtained.
Information can be recorded on such a recording medium by a pulse position modulation (PPM) or pulse width modulation (PWM) technique. A recording technique which uses PWM is also called a xe2x80x9cmark edge recordingxe2x80x9d technique.
According to the PPM recording technique, marks are recorded with the space between the marks varied, and information to be written is assigned to positions of the marks. Each of these marks is represented as a pulse with a relatively short, constant pulse width. In contrast, according to the PWM technique, marks of various lengths are recorded with the space between the marks also varied, and information to be written is represented by edge positions of the marks and spaces with a variety of lengths. Generally speaking, the density of the information recorded can be higher with the PWM technique than with the PPM technique.
In performing a PWM recording, longer marks are recorded compared to the PPM recording. If long marks are recorded on a phase-change storage medium, however, the widths of those marks might be non-uniform, because the information layers of media of this type may accumulate and dissipate heat in various manners and their recording sensitivities may be greatly different from each other. It is also known that if the information layer is continuously irradiated with radiation for a long time to record a long mark therein, then the second half of the long mark is likely to increase its width because too much heat is accumulated in that part. To avoid such an unfavorable increase in mark width, a write strategy, by which the radiation is irradiated for recording purposes as a greater number of pulses each with an even short width, was adopted. Methods and apparatuses of this type, that is to say, multi-pulse mark recording methods and apparatuses, are disclosed in U.S. Pat. Nos. 5,490,126 and 5,636,194, for example.
In order to write data onto a recording medium without any error, it is necessary to maintain an intensity of laser radiation with which the recording medium is irradiated at an appropriate level. Such an appropriate level varies depending on the kind of a recording medium. After the recording medium is inserted into a recording/reproducing apparatus, the optimization of the intensity or power level of laser radiation is automatically performed. In order to maintain the power levels of the modulated laser radiation at the target levels, however, it is necessary to always or periodically monitor the power levels of the modulated laser radiation, and to control a laser light source (a semiconductor laser) so that the optical power cannot shift from a target value.
Hereinafter, with reference to FIG. 9 and FIGS. 10(a) to (h), a prior-art semiconductor laser control circuit will be described.
FIG. 9 shows a configuration of a prior-art semiconductor laser control circuit. The control circuit controls a current for driving a semiconductor laser 1, so that laser radiation having a pulse waveform shown in FIG. 10(d) can be emitted from the semiconductor laser 1.
An intensity (power level) of the laser radiation emitted from the semiconductor laser 1 is detected and converted into a current signal by a monitoring light detector 2. The current signal is then converted into a voltage signal by a current-to-voltage converter 3. The voltage signal is input into a peak-power detector 4, a bottom-power detector 5, and a sample-and-hold circuit 6 shown in FIG. 9.
The peak-power detector 4 detects an envelope of a peak-power level in a wave-form of the input voltage signal. The bottom-power detector 5 detects an envelope of a bottom-power level. The sample-and-hold circuit 6 detects a bias-power level of laser radiation.
Outputs of the peak-power detector 4, the bottom-power detector 5, and the sample-and-hold circuit 6 are input into peak-power current controller 7, bottom-power current controller 12, and bias-power current controller 17, respectively.
The peak-power current controller 7 compares the output of the peak-power detector 4 with a predetermined reference peak-power voltage 8, and controls a value of a current flowing out of a peak-power current source 9. The current is supplied to the semiconductor laser 1 via a switch 11 which is opened and closed in accordance with a peak-power modulation signal 10 shown in FIG. 10(a).
The bottom-power current controller 12 compares the output of the bottom-power detector 5 with a predetermined reference peak-power voltage 13, and controls a value of a current flowing out of a bottom-power current source 14. The current is supplied to the semiconductor laser 1 via a switch 16 which is opened and closed in accordance with a bottom-power modulation signal 15 shown in FIG. 10(b).
The bias-power current controller 17 compares the output of the bias-power sample-and-hold circuit 6 with a predetermined reference bias-power voltage 18, and controls a value of a current flowing out of a bias-power current source 19. The current is supplied to the semiconductor laser 1 via a switch 21 which is opened and closed in accordance with a bias-power modulation signal 20 shown in FIG. 10(c).
The semiconductor laser 1 is driven by a combination of currents supplied to the semiconductor laser from the peak-power current source 9, the bottom-power current source 14, and the bias-power current source 19. The semiconductor laser 1 emits laser radiation having a waveform modulated among three levels, as shown in FIG. 10(d).
The above-mentioned control circuit involves a problem in that if a frequency of a modulation signal becomes higher, a peak-power and a bottom-power of modulated light pulses cannot be accurately detected, so that the peak-power and the bottom-power of the light pulses are deviated from their target values. Hereinafter the problem will be described.
In terms of prices, a light detector which can response at extremely high speed is not mounted on an optical head which is used in a usual optical disk recording apparatus. Thus, in the case of a light detector used in a usual optical disk recording apparatus, when a clock frequency of a modulation signal is higher, it becomes difficult to ensure frequency characteristics by which the modulation light pulse waveform in writing can be faithfully detected.
In order to enhance frequency characteristics of the light detector 2, generally, an expensive light detector with superior frequency characteristics is used, and it is necessary to focus and launch laser radiation to be monitored onto and into the light detector. The frequency characteristics of the light detector used in a usual optical disk recording apparatus are, however, not sufficient.
In the case where the frequency characteristics of the light detector 2 are insufficient, the intensity of laser radiation is detected by the light detector 2, and output from the light detector 2 as a signal having a waveform shown in FIG. 10(f). The waveform shown in FIG. 10(f) is that of a voltage signal output from the current-to-voltage converter 3. The signal waveform is substantially the same before and after the current-to-voltage converter 3.
It is understood by the comparison between FIGS. 10(d) and (f) that the output waveform of the light detector 2 does not precisely reproduce the intensity waveform of laser radiation, but has a shape obtained by blunting the intensity waveform of laser radiation.
If peak detection is performed for such an output waveform, the obtained peak-power exhibits a level of FIG. 10(e). On the other hand, if bottom detection is performed for the output waveform, the obtained bottom-power exhibits a level of FIG. 10(g). FIG. 10(e) shows an output of the peak-power detector 4, and FIG. 10(g) shows an output of the bottom-power detector 5.
If the light detector 2 has the frequency characteristics required for faithfully reproducing the waveform shown in FIG. 10(d), a peak detection output at a level 23 and a bottom detection output at a level 24 are obtained. In other words, in the case where the light detector 2 has sufficient frequency characteristics, a level difference between a peak-power and a bottom-power should have a magnitude shown by the reference numeral xe2x80x9c25xe2x80x9d. In actuality, since the frequency characteristics of the light detector 2 are insufficient, it is judged that the level difference has a relatively small magnitude shown by the reference numeral xe2x80x9c26xe2x80x9d.
When a signal having a level of FIG. 10(e) is input into the peak-power current controller 7, it is erroneously recognized that a peak-power which is excessively smaller than the peak-power 23 of the actual light pulse is detected. As a result, since the control is performed so as to increase the peak-power, the peak-power of the modulation light pulse after the power control is larger than the reference peak-power 27, as shown in FIG. 10(h). FIG. 10(h) shows an intensity waveform of laser radiation after a light power level control of laser radiation is performed by the control circuit.
When a signal having a level of FIG. 10(g) is input into the bottom-power current controller 12, it is erroneously recognized that a bottom-power which is larger than the bottom-power 24 of the actual light pulse is detected. As a result, since the control is performed so as to lower the bottom-power, the bottom-power of the modulation light pulse after the power control is smaller than the reference bottom-power 28, as shown in FIG. 10(h).
If the frequency of the modulation signal becomes higher, the frequency characteristics of the light detector are insufficient. As a result, the peak-power level and the bottom-power level of the modulated light pulse waveform cannot be precisely detected. This results in the difficulty in precise control of a semiconductor laser, and there arises a problem in that the fluctuation or error of the peak-power level and the bottom-power level of the laser light is increased.
The invention provides a semiconductor laser control circuit which can precisely control a peak-power and a bottom-power of modulated light pulses to predetermined levels, even if a light detector has insufficient frequency characteristics.
A semiconductor laser control circuit of the invention can modulate a light power level among a plurality of set levels including a first level, a second level lower than the first level, and a third level lower than the first level but higher than the second level, and the semiconductor laser control circuit includes: a first circuit portion for generating a first signal based on a light power level actually detected by a light detector in a first period in which a light power level of laser radiation is to be modulated among the first level, the second level, and the third level; a second circuit portion for generating a second signal based on a light power level actually detected by the light detector in a second period in which a light power level of the laser radiation is to be modulated between the first level and the second level; a third circuit portion for generating a third signal based on a light power level actually detected by the light detector in a third period in which the light power level of the laser radiation is to be at the third level; and a signal processor for determining a light power level of the first level and a light power level of the second level by operation, based on the first to third signals, wherein the light power level is adjusted based on an output of the arithmetic circuit portion.
A semiconductor laser control circuit of the invention can modulate a light power level between a first level and a second level lower than the first level, and the semiconductor laser control circuit includes: a first circuit portion for generating a signal indicating an average value of light intensities actually detected by a light detector in a period in which a light power level is to be modulated between the first level and the second level; a circuit portion for generating a signal indicating a light power level actually detected by the light detector in a period in which the light power level is to be at the second level; and a signal processor for obtaining a light power level of the first level, based on the two signals, wherein a light power level of laser radiation is adjusted based on an output of the arithmetic circuit portion.
A laser light source of the invention includes: the above-described semiconductor laser control circuit; and a semiconductor laser driven by the control circuit.
An apparatus of the invention includes: the above-described semiconductor laser control circuit; a semiconductor laser driven by the control circuit; and an optical system for irradiating a recording medium with laser radiation emitted from the semiconductor laser.